Certain electro-magnetic thickness measuring probes have a pot core which has a central bore, a pot chamber coaxial to the central bore accomodating a winding, and a pot rim. A pole-piece has one end fixed into the central bore and a free front-face with a coaxial recess machined therein such that an outer shell remains around the recess. A hard-metal pin is seated in the recess and has a front-face forming a crowned contact surface which stands a little proud of the pot rim.
These probes measure linearly, above all, in the region of thin coatings, avoiding the non-linearity problems which result from the magnetization curve as well as the geometry of the crowned contact surface. The probes also measure small specimens.